Recently, high speed and large capacity mutual data communication is increased accompanied with the spreading of internet and broad band communication. Moreover, the volume of data stored by government organs is rapidly enlarged with the expansion of the electronic governance promoted by Governments of many countries. Furthermore, it is expected that the needs for a photo-recording and reduction equipment and a recording medium having a high memory capacity is raised according to the spreading of high-vision and ground digital broadcasting.
Among the photo-recording-reproduction equipment having high memory capacity, a page memory system, particularly holographic recording-reproduction equipment, has been proposed as one instead of usual memory equipment. Such the equipment is noted since the system has randomly accessible high capacity memory. The holographic recording-reproduction equipment and the recording media to be used in such the equipment are described in some publications, for example, Hans J. Coufal et al. “Holographic Data Storage (Springer Series in Optical Sciences, Vol. 76)” Springer-Verlag GmbH & Co. KG, Aug. 2000.
As a holographic recording-reproduction system applied in the holographic recording-reproduction equipment, a recording-reproduction method using a holographic recording medium in which transparent substrates are arranged on both sides of a holographic recording layer such as that described in U.S. Pat. No. 5,719,691, and a recording-reproduction method using a holographic recording medium have been proposed, in which a reflective surface is arranged on one side of a holographic recording layer such as that described in Japanese Patent Publication Open to Public Inspection, hereinafter referred to as JP O.P.I. Publication, No. 2002-123949.
The holographic recording is based on the principal that the information is recorded by exposing the recording layer to holographic light for locally changing the refractive index of the recording layer, and the information is reproduced by reading the change of the refractive index in the recording layer. Various holographic recording layers are proposed, for example, one using inorganic substances such as those described in British Patent No. 9,929,953, one using substances capable of occurring structural isomerism by light such as those described in JP O.P.I. Publication No. 10-340479, and one utilizing the diffusion polymerization of photopolymer such as those described in U.S. Pat. Nos. 5,759,721 and 6,103,454. Among them, the materials using the photopolymer described in U.S. Pat. Nos. 5,759,721 and 6,103,454 have been actively investigated recently since such the material is high sensitive and the difference of the refractive index can be easily formed.
However, such the methods have a problems that the production process such as the spattering or the spin coating utilized for usual photo-recording media are difficultly applied for the holographic recording medium since the thickness of the recording layer of the holographic recording medium using the photopolymer is thicker compared with the usual photo-recording media such as CD-R, CD-RW, DVD-R, DVD-RW, DVD-RAM and DVD-ROM, and is usually more than 200 μm.
Moreover, in the photo-recording medium using the photopolymer, it is effective to raise the mobility of the compound in the recording layer for making the difference between the refractive index of the holographically exposed area and that of the unexposed area. However, when the mobility of the compound in the layer is raised, the medium shows a drawback such as that formation of the unevenness of thickness of the recording layer of the photo-recording medium is difficultly inhibited and that the portion having the different refractive index localized by the holographic exposure is easily moved in the recording layer.
For improving such the drawback, a method by arranging a supplemental member between the substrates constituting the recording medium is proposed, for example, in U.S. patent application Ser. No. 2002/0145772. Such the method tends to be insufficient for preventing the difference of layer thickness between each of the recording media even though the method is effective for preventing the positional discrepancy of the substrates.